The present invention is directed to a backup mandrel. More particularly, the present invention is directed to a backup mandrel for attaching a grommet to a washer within an opening in an aircraft panel.
An aircraft typically has an outer skin comprised of a plurality of panels. Each panel, in turn, typically comprises a plurality of openings for receiving screws, bolts, rivets or the like (hereinafter fasteners), which are used to fasten the panel to an internal structure of the aircraft. When the panel has been fastened to the internal structure of the aircraft in this way and the panel experiences a force tending to shift the panel, the fasteners exert a corresponding force on the panel openings to prevent the panel from shifting. However, the fasteners are generally comprised of a material that is much stronger than the material of the panel. For example, a typical fastener is comprised of stainless steel or titanium, while a typical panel is comprised of a composite material, such as fiber glass. Consequently, as the fasteners bear against the panel openings, over time the panel openings wear, i.e. become enlarged, resulting in a loose connection between the panel and the internal structure of the aircraft, or worse, the fasteners fracture or delaminate the panel, possibly resulting in the panel disengaging from the internal structure of the aircraft.
To alleviate this problem a metal insert may be affixed within the panel opening. For example, FIG. 1A shows a metal grommet 10 inserted into an aircraft panel 12 through a panel opening 14. A first end 10A of the grommet 10 abuts a first surface 12A of the panel 12, while a washer 16 having an opening 160 abuts a second surface 12B of the panel 12. The grommet 10 extends through both the panel opening 14 and the washer opening 160, such that a second end 10B of the grommet 10 protrudes from the washer opening 160. To affix the grommet 10 within the panel opening 14, the second end 10B of the grommet 10 is swaged, i.e. flared or bent, towards a recessed area 16R in the washer 16 until the second end 10B of the grommet 10 abuts an exposed surface of the washer 16 within the recessed area 16R, as shown in FIG. 1B.
FIGS. 2A-2C show prior art backup mandrels 18, 18A and 18C having shoulders of varying lengths. For clarity and to avoid duplication, a majority of the following description will involve the backup mandrel 18 having a shoulder 21B. The backup mandrel 18 comprises a nose piece 20 having the shoulder 21B and a mandrel stem 22. The mandrel stem 22 comprises a stem head, a stem body and a stem tip. The stem head is larger than a nose piece opening and the stem body and stem tip are smaller than the nose piece opening to allow the stem body and tip to be inserted into the nose piece opening. A first portion of the stem body is housed within the nose piece opening, while a second portion of the stem body and the stem tip protrude from the nose piece opening.
FIGS. 2D-2F show the prior art backup mandrel 18 being used to affix the grommet 10 within the panel opening 14. In use, the stem tip, the stem body and the shoulder 21B are inserted into the grommet 10 until the nose piece 20 abuts the first end 10A of the grommet 10 and the stem tip and the second portion of the stem body protrude from the second end 10B of the grommet 10. The second portion of the stem body and the stem tip are then inserted into an opening in a swage gun 24. The swage gun 24 comprises a first swage tool 28, which is moved in a position disposed within an inner diameter of the grommet 10. Within the swage gun 24 is a plurality of jaws 26 having teeth that removably clampingly engage a plurality of grooves 25 in the stem tip. When the jaws 26 are clampingly engaged with the stem tip, the jaws 26 pull the stem, and therefore the entire backup mandrel 18 towards the swage gun 24. This motion causes the second end 10B of the grommet 10 to contact an angled shoulder 30 on the first swage tool 28.
As shown in FIG. 2E, as the second end 10B of the grommet 10 is pressed against the angled shoulder 30 of the first swage tool 28, the second end 10B of the grommet swages, i.e. is bent, in the shape of the angled shoulder 30. However, as can be seen in FIG. 2E, during the swaging, a portion 32 of the inner diameter of the grommet 10 is not supported. As a result, buckling 34 often occurs in the unsupported portion 32 of the grommet 10. As a result, as the jaws 26 continue to pull the backup mandrel 18 towards the swage gun 24, the shoulder 21 is often compressed by the buckling 34. This makes removal of the shoulder 21B from the grommet 10 after the grommet 10 has been affixed to the panel 12 difficult. In addition, the compression of the shoulder 21B can cause breakage of the mandrel stem 22; tearing or delaminating of the panel 12; or failure of the installed grommet. Also, in some instances the buckling 34 reduces the grommet internal diameter to the point that the fastener intended to be inserted into the grommet 10, to attach the panel 12 to the internal structure of the aircraft, cannot be inserted through the grommet 10 past the buckling 34. In such an instance, the grommet 10 must be drilled out of the panel 12 and, if the panel 12 did not suffer any damage, a new grommet 10 must be installed into the panel 12.
As shown in FIG. 2F, when a biasing force that biases the first swage tool 28 towards the grommet 10 is overcome by the force required to further swage the grommet 10, a further pulling of the backup mandrel 18 towards the swage gun 24 by the jaws 26 causes the first swage tool 28 to retract into the swage gun 24. When the first swage tool 28 is retracted, a second swage tool 36 is allowed to contact the second end 10B of the grommet 10. Thus, as the jaws 26 continue to pull the backup mandrel 18 towards the swage gun 24, the second swage tool 36 contacts the second end 10B of the grommet 10, swaging the second end 10B of the grommet 10 towards the recessed area 16R of the washer 16 until the second end 10B of the grommet 10 contacts the washer 16.
However, because the backup mandrel 18 does not comprise a force limiting or shock absorbing means, the swaging of the grommet 10 by the first and second swage tools 28 and 36 occurs at full force and very abruptly, i.e. the first and second swage tools 28 and 36 slam into the second end 10B of the grommet 10 at full force and with considerable shock, causing the first end 10A of the grommet 10 to forcefully press against the nose piece 20, often causing dishing 38 (see FIG. 3C) in the first end 10A of the grommet 10.
The dishing 38 often causes the fastener, which is intended to be inserted into the grommet 10 to attach the panel 12 to the internal structure of the aircraft, to not fit flushly against the first end 10A of the grommet 10. As a result, it is difficult to fasten the fastener as far into the internal structure of the aircraft as desired, resulting in a loose connection of the panel 12 to the internal structure of the aircraft. Also, dishing 38 usually causes a collapse of the internal diameter at 10A end of the grommet 10, requiring the grommet to be removed from the structure, and replaced.
The backup mandrel 18 also has other disadvantages. For example, if the shoulder 21B contacts the first swage tool 28 the shoulder 21B will cause the first swage tool 28 to retract into the swage gun 24. As such, the length of shoulder 21 is dimensioned such that it does not contact the first swage tool 28 until the first swage tool 28 has swaged the second end 10B of the grommet 10 by a desired amount. Therefore, the length of the shoulder 21B is designed such that it is as long as possible without interfering with the first swage tool 28 during swaging. Consequently, each backup mandrel 18, 18A and 18C comprises a shoulder 21 that makes the backup mandrel 18 useful for only a limited range of panel 12 thicknesses, or in some cases for only one panel thickness. For example, if a panel has a thickness less than the thickness of the panel 12 shown in FIGS. 2E-2F, then the backup mandrel 18 cannot be used because the shoulder 21B is too long, i.e. the shoulder 21B will contact the first swage tool 28, thus forcing the first swage tool 28 to retract before the second end 10B of the grommet 10 has been swaged to a desired amount, thus resulting in a loose connection between the panel 12 and the grommet 10. Therefore, as shown in FIGS. 2G and 2H, very thin panels require the backup mandrel 18A, which has no shoulder and in some cases has a recessed counterbore 21A. The absence of a shoulder on this backup mandrel 18A makes buckling 34 even more likely to occur. FIG. 3A shows the buckling 34 that typically occurs by use of the backup mandrel 18, while FIG. 3B shows the buckling 34 that typically occurs by use of the backup mandrel 18A.
The present invention addresses the above-referenced problems by providing a backup mandrel for attaching a grommet to a washer in an aircraft panel opening comprising a housing and a nose piece attached to the housing, the nose piece having an opening. A stem is attached to the housing and has a first and a second portion, such that the first portion is disposed within the housing and the second portion protrudes from housing through the nose piece opening. A sleeve is also attached to the housing and has a first and a second segment, such that the first segment is disposed within the housing and the second segment protrudes from the housing through the nose piece opening, wherein the sleeve is displaceably mounted around the stem.
In addition, the present invention addresses problems of the prior art by providing a method of attaching a grommet to a washer in an aircraft panel opening comprising providing an aircraft panel having an opening; inserting a grommet into the panel opening, such that a first end of the grommet abuts a first side of the panel and a second end of the grommet protrudes from a second side of the panel, the grommet having an inner diameter; placing a washer over the protruding portion of the grommet, such that a first side of the washer abuts the second side of the panel and the second side of the washer is exposed; providing a back up mandrel having a nose piece attached to a housing, wherein a stem and sleeve are attached to the housing and protrude from an opening in the nose piece; inserting the protruding portions of the stem and sleeve into the inner diameter of the grommet; providing a swage gun having first and second swage tools; activating the swage gun, such that the swage gun exerts an axial force on the backup mandrel stem, causing a relative movement between the grommet and the swage tools; continuously supporting the entire inner diameter of the grommet as the grommet is swaged by the first and second swage tools.